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Past, present and future of hard X-ray/γ-ray measurements of GRB polarization

Past, present and future of hard X-ray/γ-ray measurements of GRB polarization. P. Laurent CEA/DSM/IRFU/SAP & APC. June , 13 th 2012. Incident photon. e. Scattered photon. Compton polarimetry principles.

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Past, present and future of hard X-ray/γ-ray measurements of GRB polarization

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  1. Past, present and futureof hard X-ray/γ-ray measurementsof GRB polarization P. Laurent CEA/DSM/IRFU/SAP & APC June, 13th 2012

  2. Incident photon e Scattered photon Compton polarimetry principles • Compton scattering cross section is maximum for photons scattered at right angle to the direction of the incident electric vector  asymmetry in the azimuthal profile S of scattered events. • modulation • a = modulation factor • polar. fraction = PF = a/a100 • a100 = modulation for a 100 % polarized source. • polar. angle = PA = 0 - /2 + n GRB polarisation observations June, 13th 2012

  3. History of GRB polarization observations • The past : RHESSI • The present : Integral and GAP • The future : POLAR, NuStar, Astro-H, … GRB polarisation observations June, 13th 2012

  4. GRB RHESSI observations • The RHESSI spectrometer. • Observation of polarization in GRB 021206. • Controversy on this measure. GRB polarisation observations June, 13th 2012

  5. The RHESSI spectrometer • Launched in 2002 to study solar activity. • 9 germanium detector with modulated coded mask. • Could measure polarization between 20 and 2 MeV. • a100 ~ 40 % at 50 keV. Mc Connell et al., 2002, Solar Phys. 210, 125 GRB polarisation observations June, 13th 2012

  6. RHESSI observation of GRB021206 GRB 021206 : a bright GRB detected up to 2 MeV. Data analysis :unpolarized signal simulated by Monte-Carlo and subtracted to real data. Coburn & Boggs, Nature, 2003, 423, 415 25 – 150 keV 150 – 500 keV 500 keV – 2 MeV 150 keV – 2 MeV GRB polarisation observations June, 13th 2012

  7. Controversy • Data reanalyzed in 2004 by Rutledge & Fox (MNRAS 350, 1288 (2004) : • Real double events number is a factor 10 below than this estimated by Coburn & Boggs  lower S/N. • Polarization signal is in fact dominated by systematics. •  Difficulty of polarization data analysis GRB polarisation observations June, 13th 2012

  8. OMC (optical) JEM-X Satellite 4.1 tons 5 m height 3.7 m diameter Launched in 2002 INTEGRAL Scientific payload ISDC IBIS 15 keV – 10 MeV 12’ FWHM imaging <1’ source location 19°x19° FOV SPI 20 keV – 8 MeV 2 keV FWHM 26° Ø FOV

  9. Integral GRB observations • Polarimetry with the Integral IBIS/Compton telescope. • Polarimetry with the Integral/SPI telescope. • Observation of variable polarization in GRB 041219A. • Constraints on LIV from GRB 041219A observations. GRB polarisation observations June, 13th 2012

  10. The IBIS telescope GRB polarisation observations June, 13th 2012

  11. Incident photon e Scattered photon The IBIS/Compton telescope • The IBIS telescope is a coded mask telescope which could be used as a Compton telescope. • The Compton mode events are ISGRI and PICSIT events in temporal coincidence, within a window W  3.8 s. ISGRI PICsIT • Within this window, chance coincidence, called hereafter “spurious events”, may also occur. GRB polarisation observations June, 13th 2012

  12. The a100 factor • a100 necessary to estimate the pulse fraction. • a100 estimate: GEANT 3/ GLEPS simulation for a 100 % linearly polarized source. • a100 = 0.304 ± 0.003 for a Crab-like spectrum • No on-ground calibration. GEANT 3 simulation: a100 between 0.2 and 0.4 GRB polarisation observations June, 13th 2012

  13. Data analysis summary • Spurious events correction • Uniformity correction • Coded mask deconvolution GRB polarisation observations June, 13th 2012

  14. Spurious correction “SPURIOUS EVENTS” 1 ISGRI event + 1 independent PICSIT event detected during the coincidence window False source detection 1. We compute the spurious events contribution: NSPUR/NISGRI ~ WNPICSIT 2. We compute “fake” spurious events, composed of one ISGRI single event randomly associated to one PiCsIT single event. 3. We build sky image with these events that we subtract from the Compton ones. GRB polarisation observations June, 13th 2012

  15. Compton imaging:Non-uniformity corrections Compton/ISGRI image Uniformity map Uniformity map deconvolved Uniformity profiles GRB polarisation observations June, 13th 2012

  16. Image deconvolution Shadowgram deconvolution 200-800 keV T=300 ks shadow  SOURCE DIRECTION GRB polarisation observations June, 13th 2012

  17. Checks for systematics maximum modulation for unpolarized data? (square detectors, grids, pixels, mask pattern…) • strong calibration source • at 392, 511, 662 keV • a < 5-7 % • empty fields • a < 5 % • observations at ≠ source-detector angles • same results • spurious event files • a = 15 % @ 180° Spurious events GRB polarisation observations June, 13th 2012

  18. Polarimetry with the SPI telescope GRB polarisation observations June, 13th 2012

  19. Polarimetry with SPI SPI has also be used as a Compton telescope using multiple events in the Germanium detectors j - angle between incident photon polarisation direction and scattered photon direction.  GRB polarisation observations June, 13th 2012

  20. Polarimetry with SPI Polarimetry with SPI is determined using a very detailled Mass Model (Dean 08) • Photons with the same spectrum and direction as the source under investigation are simulated interacting with a detailed model of SPI and the surrounding spacecraft. • The energy deposits can be analysed in the same manner as for the real instrument • The flux is modelled with different angles of polarisation • Then compared to the real data taken by the instrument. GRB polarisation observations June, 13th 2012

  21. 18 polarised beams for each azimuthal angle in 10º steps between 0º and 170º (180º symmetry) + 1 non-polarised • ~700,000 singles & ~70,000 doubles produced for each pointing • Polarised & unpolarised simulated data combined to produce any percentage polarisation () needed using: P100 = 100% polarised P0 = unpolarised P% = percentage polarised data Polarimetry with SPI • Mass Model simulation of Crab spectrum for each pointing GRB polarisation observations June, 13th 2012

  22. Polarimetry with SPI • Data fitted on a Science window by Science window basis • Each adjacent detector pairs considered (Pseudo detectors: 42 later reduced to 32 after failure of the two Ge pixels) • Recorded data from Crabmodelled as: • S = the Crab strength • C = count distribution from the simulation • Bi = spatial distribution of the background • Bs = background variation in time GRB polarisation observations June, 13th 2012

  23. Study of polarisation of GRB 041219A GRB polarisation observations June, 13th 2012

  24. GRB 041219A in a few words • GRB 041219A was detected in december 2004 by the Integral Burst Alert System (IBAS). • Longest and brightest GRB detected in the Integral FOV so far … • The Integral Spectrometer (SPI) already reported a high polarisation level (68 %) observed during the brightest part of this GRB (Mc Glynn et al., 2007) GRB polarisation observations June, 13th 2012

  25. GRB 041219A Compton light curve Analysis in 10s bins Total S/N (200-800 keV) 37  5s bin GRB polarisation observations June, 13th 2012

  26. GRB 041219A polarisation diagrams = SPI GRB polarisation observations June, 13th 2012

  27. GRB 041219A polarisation results = SPI SPI : 63 ± 30 % P.A. = 70 ± 12° GRB polarisation observations June, 13th 2012

  28. GRB 041219A: constraints on LIV • LIV : Lorentz invariance violation • LIV => rotation of the polarization angle • Already studied using the SPI measurement of Crab polarization (Maccione et al. 2008) GRB polarisation observations June, 13th 2012

  29. GRB 041219A: constraints on LIV • Principle: • Light dispersion relation: MPl: reduced Planck scale (2.4 1018GeV) GRB polarisation observations June, 13th 2012

  30. GRB 041219A: constraints on LIV Crab:  < 2 10-9 (Maccione et al. 2008) GRB : at least 105 times furtheraway GRB polarisation observations June, 13th 2012

  31. GRB 041219A: distance determination • Redshift measurewith the CFHT/WirCAM instrument (Götz et al., 2011): • => z = 0.31 +0.54 -0.26 • => d = [0.222-5.406] Gpc • with standard cosmologicalparameters • (m=0.3, =0.7, H=70 km/s/Mpc) GRB polarisation observations June, 13th 2012

  32. GRB 041219A: measure of  Comparaison of PA between 2 energy bands with similar signal to noise 68% 90% 95% 99% proba(a > a0, any ) = 0.06 % GRB polarisation observations June, 13th 2012

  33. GRB 041219A: constraints on LIV Comparaison of PA between 2 energy bands 200-250 keV vs 250-325 keV  = 21 ± 47°  < 4 10-15 GRB polarisation observations June, 13th 2012

  34. IKAROS/GAP GRB observations GRB polarisation observations June, 13th 2012

  35. The IKAROS/GAP experiment • Japanese experiment launched in2010. • GAP : Gamma-Ray Burst polarimeter on the solar sail IKAROS. • Plastic – CsI axial Compton telescope. • Heavily tested on ground in polarized beams. GRB polarisation observations June, 13th 2012

  36. GAP observation of GRB 100826A • GRB 100826A observed by GAP (70 – 300 keV). • Modulation fitted with a Monte-Carlo model. • Marginal detection at 2.9  GRB polarisation observations June, 13th 2012

  37. FUTURE MISSIONS … GRB polarisation observations June, 13th 2012

  38. The POLAR telescope (2014) • POLAR is a Swiss lead mission to be placed on the Chinese space station Tiangong 2 (2014). • It is a Compton telescope dedicated to GRB polarization measures. • It consists of several bars of plastic scintillator readout by PMT (a100 ~ 60 %). GRB polarisation observations June, 13th 2012

  39. Astro-H (2014) Astro-H :next X-ray Japanese mission • 4 instruments including a Compton telescope (SGD, 5-600 keV). • Good polarimetric results on ground. • But, FOV (10°), delimited at high energy (> 300 keV) by BGO collimators. 250 keV X-ray beam SGD telescope GRB polarisation observations June, 13th 2012

  40. NuStar (2012) NuStar :new hard X-ray focusing telescope (6-80 keV) to be launched today (20h30 Moscow time). • A mirror focuses X-rays toward a CZT detector, 12 m away. • Polarization measures could be made by studying Compton scatter between different CZT pixels. • But, small FOV  GRB scarcely observed. GRB polarisation observations June, 13th 2012

  41. ASTROSAT (2012), UFFO-100 (2015), SVOM (2017), … ASTROSAT :next X-ray/UV indianmission UFFO-100 :russian/korean mission SVOM :French/Chinese GRB mission hard X-ray wide field imager: CZT, LSO detector + coded mask which may be also used as “90° polarimeters”. GRB polarisation observations June, 13th 2012

  42. Conclusion • The measure of polarization in hard-X/soft-gamma rays is a powerful tool to investigate the emission mechanisms and geometry of Gamma-Ray Bursts. • Fundamental physics questions can also be addressed • Next generation polarimeters (e.g., POLAR, Astro-H, etc.) will complement the present discoveries ! • Several Compton telescope R&D projects are on-going all over the world dedicated to hard X-ray polarimetry measurement. GRB polarisation observations June, 13th 2012

  43. Thankyou ! GRB polarisation observations June, 13th 2012

  44. Interpretation(s) • synchrotron emission from shock accelerated electrons in a relativistic jet with magnetic field transverse to the jet expansion (Granot 2003, Granot & Königl 2003, Nakar, Piran & Waxman 2003) (ii) synchrotron emission from purely electromagnetic flow (Lyutikov et al. 2003, Nakar, Piran & Waxman 2003) (iii) synchrotron emission from shock accelerated electrons in a relativistic jet with a random magnetic field (Ghisellini & Lazzati 1999, Waxman 2003) SAME POLARIZATION LEVELS AS IN (I) BUT A PECULIAR OBSERVATION CONDITION IS NEEDED (Θobs ≅ Θjet+k/Γ) GRB polarisation observations June, 13th 2012

  45. Interpretation(s) (iv) Inverse Compton scattering from relativistic electrons in a jet propagating in a photon field (“Compton drag”) (Lazzati 2004) POLARIZATION LEVELS can reach 60-100% BUT ONLY UNDER THE CONDITION OF A NARROW JET (ΓΘjet<5) AND THE SAME OBSERVATION CONDITIONS AS IN (iii) APPLY (v) Independently from the emission process (synchrotron or inverse Compton), fragmented fireballs (shotguns, cannonballs, sub-jets) can produce highly polarized emission, with a variable P.A. The fragments are responsible for the single pulses and have different Lorentz factors, opening angles and magnetic domains. (e.g. Lazzati & Begelman 2009) GRB polarisation observations June, 13th 2012

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